Polly policy to log exception and rethrow

Question

I consider to use Polly to create policy to log exception and rethrow. I didn't find an existing method that allow it out of the box , but some options that I see are

Fallback

// Specify a substitute value or func, calling an action (e.g. for logging)
// if the fallback is invoked.
Policy.Handle<Whatever>()
.Fallback<UserAvatar>(UserAvatar.Blank,
    onFallback: (exception, context) =>
    {
        _logger.Log(exception, context);
        throw exception;
    });

Question: is it ok to throw exception from Fallback?

Timeout

Policy.Timeout(1, T30meoutStrategy.Pessimistic,
(context, timespan, task) =>
{
    // ContinueWith important!: the abandoned task may very well still be executing,
    // when the caller times out on waiting for it!
    task.ContinueWith(t =>
    {
        if (t.IsFaulted)
        {
            logger.Error(context,t.Exception);
            throw exception;
        }
    });
}

Or Retry

Policy.Handle<DivideByZeroException>().Retry(0,
(exception, retryCount) =>
{
    logger.Error(context,exception);
    throw exception;
});

Question: is 0 retries supported?

Or KISS and write simple try/catch with throw by myself.

Which of these methods is better? What are your recommendation?

Answer 1

If you do not already have Polly in the mix, try/catch would seem simplest.

If you already have Polly in the mix, FallbackPolicy can safely be re-purposed in the way you suggest. The onFallback delegate and fallback action or value are not governed by the .Handle<>() clauses of the Policy, so you can safely rethrow an exception from within the onFallback delegate.

Policy<UserAvatar>.Handle<Whatever>()
.Fallback<UserAvatar>(UserAvatar.Blank,
    onFallback: (exception, context) =>
    {
        _logger.Log(exception, context);
        throw exception;
    });

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The approach your question outlines with TimeoutPolicy would only capture exceptions thrown by delegates the caller had earlier walked away from due to timeout, and only in TimeoutMode.Pessimistic; not all exceptions.

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The approach your question outlines with .Retry(0, ...) would not work. If no retries are specified, the onRetry delegate would not be invoked.

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To avoid the untidiness of repurposing FallbackPolicy, you could also code your own LogThenRethrowPolicy, within Polly's structures. This commit (which added the simple NoOpPolicy) exemplifies the minimum necessary to add a new policy. You could add an implementation similar to NoOpPolicy but just try { } catch { /* log; rethrow */ }

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EDIT January 2019: Polly.Contrib now also contains a Polly.Contrib.LoggingPolicy which can help with this.